FR2665991A1 - DUAL MODE FREQUENCY CONTROL SYSTEM. - Google Patents

Abstract

A frequency control system (152-222) is provided for locking the frequency of a receiver, such as the receiver part of a transceiver, to the frequency of a transmitter, such as a radio station. base. The frequency control system is able to determine the center frequency, or other reference frequency, of a signal which is either a conventional analog signal or a discrete encoded signal. The frequency control system can advantageously be implemented in a dual mode radiotelephone having the function of receiving both conventional analog signals and discrete coded signals.

Description

The present invention relates generally to a

  automatic frequency control apparatus and, more particularly

  an automatic frequency control system for correcting frequency differences between a receiver and a transmitter which transmits an analog information signal or a signal

discrete coded information.

  An information signal is superimposed on an electro- wave.

  magnetic by a process called "modulation" In a process of

  modulation, the information signal is combined with an electric wave

  tromagnetic (called the carrier wave), and the resulting combined signal is an electromagnetic wave which varies in a certain way according to the values of the information signal Various modulation techniques have been developed to modulate the information signal on the electromagnetic wave; amplitude modulation, frequency modulation and phase modulation

  are three of these modulation techniques.

  Generally, an amplitude modulated signal is formed by modulating the information signal on the electromagnetic wave so that the information signal changes the amplitude of the wave

  electromagnetic depending on the value of the information signal.

  The frequency of the electromagnetic wave does not vary, and the content

  in information of the modulated signal is contained in the form, or am-

  signal amplitude The signal shape is called the signal envelope, and the variations in amplitude of the modulated signal modify

  the envelope formed by this one forms a signal modulated in fr

  quence by modifying the frequency of the electromagnetic wave as a function of the value of the information signal The amplitude of the electromagnetic wave does not vary, and the information content of the modulated signal is contained in the variation of the signal frequency A phase modulated signal is formed by modifying the phase of

  the electromagnetic wave as a function of the value of the signal of in-

  formation The amplitude of the electromagnetic wave does not vary, and the information content of the modulated signal is contained in the variation of the phase of the signal Since the amplitude of a frequency modulated signal and that of a signal modulated in do not vary, these modulated signals are referred to as

constant envelope.

  A receiver which receives the modulated information signal comprises a circuit making it possible to detect, or else, if not,

  recreate, the information signal modulated on the electromagnetic wave

  This process is called demodulation, and various receiving circuits

  allow to demodulate the information signals modulated on

  an electromagnetic wave according to the various modulation techniques

  lation. Many different modulated information signals can be transmitted simultaneously using several

  transmitters operating on several different frequencies.

  Parts of a 100 MHz band of the frequency spectrum

  electromagnetic frequency (that part extending between 800 M Hz and 900 M Hz) are allocated to the transmission by radiotelephone, for example by means of radiotelephones employed in a cellular transmission system A radiotelephone contains the circuits

  allowing both to create and receive information signals

modulated mation.

  We’ve created a cellular transmission system

  many base stations at locations separate from a geographic area Each of the base stations is designed

  to simultaneously receive modulated information signals pro-

  coming from radiotelephones and transmitting them to them,

  in a way that ensures a two-way transmission between them

  The base stations are placed in places such that a radiotelephone located at any point in the geographical area is in the reception area of at least one

  base station receivers.

  The geographic area is divided into parts, and a base station is placed in each part Each part of

  the geographic area thus defined is called a "cell".

  As indicated above, part of the band of fre-

  100 MHz frequency is allocated to cellular communications.

  While many modulated information signals can be transmitted simultaneously at different transmission frequencies, each occupies a finite part of the frequency band.

  of modulated information signals transmitted simultaneously is not self-

  risky because interference between overlapping signals on the same frequency could prevent a receiver from detecting one

  or the other of the modulated information signals.

  The frequency band is divided into channels, which each have a bandwidth of 30 k Hz Currently, it is allowed to transmit a single signal in each channel of 30 k Hz wide in the frequency band In addition, a first part, extending between 824 M Hz and 849 M Hz, of the frequency band is allocated to the transmission of modulated information signals from a radiotelephone to a base station A second part, extending between 869 M Hz and 894 M Hz, the frequency band is allocated to the emission of

  modulated information signals from a base station to a radio

  telephone In the first part of the frequency band, 832 transmission channels are formed and, in the second part of the

  frequency band, 832 transmission channels are formed, which automatically

  make a maximum of 832 simultaneous two-way communications

  within the limits of a geographical area.

  A modulated signal transmitted on any of the transmit channels must have a Bandwidth less than the width

  bandwidth of the transmit channel (i.e., less than 30 K Hz).

  Lators' oscillations which oscillate at frequencies allowing

  duire the electromagnetic carriers are likely to know

  be frequency variations These variations, which are called frequency drifts, can cause the transmitted signal to extend

  beyond the borders of the broadcast channel.

  The increased use of cellular transmission systems

  in many cases led to full use

  of all transmission channels allocated for radio transmission

  cell diotelephone Other frequency bands of the electromagnetic spectrum are, similarly, often used entirely. Various attempts have been made to increase the information transmission capacity of cellular radiotelephone transmission systems as well as other transmission systems using other frequency bands of the spectrum.

  electromagnetic However, radio transmission systems

  cellular telephones consist of radiotelephones and base stations having circuits which transmit and receive analog signals modu Frequency controlled It is possible to transmit on a channel of transmission only one signal of information modulated at a time Significant increase in transmission capacity

  information systems of radiotelephone transmission systems

Lular is therefore limited.

  However, "discrete" modulation techniques have been developed which allow the emission of more than one signal at the same frequency. A cellular radiotelephone transmission system which is capable of transmitting information signals

  modules formed by discrete modulation techniques allow

  milk to transmit more than one signal on a transmission channel La

  capacity of such a transmission system can then be notable-

increased.

  In general, a discrete modulation technique encodes a continuous information signal into "discrete" signals, then modulates the discrete signals on an electromagnetic wave so as to thus form a modulated information signal. The discrete signals of more than an information signal can be modulated on

  electromagnetic waves having identical carrier frequencies

  ticks and be issued sequentially to two radio stations

phones or more.

  Frequency drift can be a more serious problem.

  serious for the emission of certain discrete coded signals The os-

  cillators which produce the electromagnetic waves on which

  the information signals are modulated are likely to

  present a certain frequency drift as a function of varia-

  environmental conditions, such as those of the

  supply temperature or voltage A drift of

  quence of a range retaining a conventional analog signal between the borders of the transmission channel can often be such that it brings a discrete coded signal corresponding to pass beyond the borders of the transmission channel The frequency drift of signals d information produced by discrete modulation techniques is more likely to experience interference problems than is the case for modulated information signals that are produced by

classic analog modulation.

  In addition, the circuits used to demodulate transmitted information signals corresponding to certain coded type signals

  discrete require less frequency error than that allowed

  for conventional analog signals In quantitative terms

  according to the standard of the United States of America, the error of

  quence of a cellular system which is authorized for conventional analog signals is 2.5 parts per million, while the frequency error authorized for discrete coded signals is

about 0.2 parts per million.

  We know, and we frequently use them in many -

  breux existing transmission systems, systems and pro-

  frequency control assignments to minimize frequency drift and, therefore, minimize problems related to

  frequency drift In general, we adjust a single bone

  cillator, called the reference oscillator, which is placed in a transmitter, so that the frequency drift of the signal produced

  by it is included within an admitted interval-

  sible We can then lock the other oscillators of the system

  radio on the frequency of the reference oscillator.

  In the particular case of radiotelephone transmissions

  cell nics, as above indicated, it is possible to precisely control the oscillators of the base stations placed in the geographical area and used to modulate an information signal, so as to minimize the frequency drift of the electromagnetic wave they produce The receptors of

  radiotelephones can use the frequency of the information signal

  modulated from the base station as a reference frequency

  The reference frequency is used by the radiotelephone.

  does, for example, as a reference from which the

  frequency of radiotelephone transmission can be shifted to allow the signal transmitted by the radiotelephone to be as precise

  in frequency than The base station.

  To increase the capacity of cellular transmission systems, existing base stations with circuits

  which only transmit and receive conventional analog signals

  must be converted to base stations which authorize

  also feels the transmission and reception of discrete coded modulated information signals. Radiotelephones are being developed for the transmission and reception of signals.

  classic analog as well as discrete coded signals.

  Since we gradually convert the base stations of a system

  cellular and that we develop in the same way

  telephones, certain channels of a cell may be

  killed by receivers that have circuits that allow reception of discrete coded modulated information signals, while other channels will include receivers that have circuits that allow only reception of information signals

  classic analog modulated Similarly, some radio-

  telephones used in cellular transmission systems will contain circuits which allow the transmission and reception of discrete coded modulated information signals as well as conventional analog modulated information signals Other radiotelephones will contain circuits which do not allow transmission and reception of analog modulated information signals

classics.

  You can build a dual mode radiotelephone.

  risking the emission of conventional analog modulated information signals as well as discrete coded modulated information signals, which has both a first transmission circuit and

  for receiving analog modulated information signals c Las-

  sic and a second circuit for transmitting and receiving discrete coded modulated information signals When a radiotelephone receives a discrete coded signal, a digital signal processor can conveniently be used to decode the signal. digital signals can be used to produce an error signal to correct the frequency of

  reference used in the radiotelephone.

  While a digital signal processor can be used to generate the error signal to correct the reference frequency when receiving an analog signal, this digital signal processor dissipates greater amounts of energy than the conventional analog circuit used to determine the reference frequency of a conventional analog modulated information signal However, the conventional analog circuit is not satisfactory with regard to the determination of the reference frequency associated with

  discrete coded information signals.

  A radiotelephone capable of receiving both conventional analog modulated and discrete coded information signals, which

  has a circuit for determining the reference frequency

  rence of both types of signal emitted and which, in addition, has

  minimum energy consumption requirements would be preferred.

  geux. So what you need is a frequency control diagram

  which requires minimal energy consumption, but can also

  It can also be used to determine the reference frequency of conventional analog modulated information signals or discrete coded modulated information signals which are transmitted by the radiotelephone. It is therefore an object of the invention to produce a frequency control system making it possible to correct the frequency differences between a transmitter and a receiver having the function of transmitting and receiving both analog modulated information signals and coded modulated information signals

discreet.

  Another object of the invention is to produce a system of

  frequency control whose function is to determine the frequency

  reference quence of a signal transmitted either by techniques of

  classic analog modulation, either by modu-

  discrete lation, which system requires minimum consumption

of energy for its operation.

  Another object of the invention is to produce a radio

  dual mode telephone having the function of receiving both a conventional analog information signal and a discrete coded signal, which has a frequency control circuit having

  minimum power consumption requirements.

  It is therefore described, according to the invention, a frequency control system making it possible to correct frequency differences between a receiver having the function of receiving an information signal modulated on a carrier wave characterized by a certain frequency in order to thus form a modulated information signal, and a transmitter which transmits the information signal The modulated information signal is received, and an analog signal detection circuit determines the particular frequency characterizing the carrier wave when the information signal is an analog signal, and produces a first frequency reference signal which is indicative of the particular frequency thus determined. A discrete signal detection circuit determines the particular frequency characterizing the carrier wave when the information signal is a discrete coded signal, and produces a second frequency reference signal which is indicative of the frequency carrier frequency thus determined The oscillation frequency of a variable frequency oscillator is then modified as a function of the values of the first frequency reference signal or the second frequency reference signal, so as to thus correct the differences between the

receiver and transmitter.

  The following description, intended as an illustration of

  the invention aims to give a better understanding of its characteristics and advantages; it is based on the appended drawings, among which: FIG. 1 is a graphic representation of an amplitude modulated information signal which is representative of one of the signals which can be used by the frequency control system se Lon the invention; FIGS. 2 A and 2 B are graphical representations of signals with constant envelope, FIG. 2 A being a frequency modulated signal representative of one of the signals which can be used by the frequency control system according to the invention, and FIG. 2B being a phase modulated signal representative of another signal which can be used by the frequency control system according to the invention;

  FIG. 3 is a graphical representation of the cons-

  telling points of a discrete coding scheme which can be used to code an information signal so as to form a discrete coded signal; Figure 4 is a graphical representation of the frequency modulated signal of Figure 2 A, shown as a function of frequency; Figure 5 is a graphical representation of a signal

  DQPSK (modulation by differential quadrivalent phase displacement

  tielle), which is a combination of an amplitude modulated signal and

  of a phase modulated signal, represented as a function of the frequency

  quence; FIG. 6 is a graphic representation of two adjacent transmission channels of a frequency band, where a conventional modulated information signal is transmitted on a first of the transmission channels and a discrete coded modulated information signal is transmitted on the second of the transmission channels; Figure 7 is a block diagram of the frequency control system according to the invention; Figure 8 is a representation, partially in block diagram and partially in circuit form, of a preferred embodiment of the invention; and FIG. 9 is a representation, partly in the form of a block diagram and partly in the form of a circuit,

  of another preferred embodiment of the invention.

  We first refer to the graphical representations of FIGS. 1 and 2 A-2 B, which show waveforms representative of three types of modulated information signals Waveforms

  similar to those of Figures 1 and 2 A-2 B (or, more particularly-

  ment, a waveform analogous to the waveform in Figure 2 A, and a waveform analogous to the combination of Figures 1 and 2 B)

  can be used by the system according to the invention to correct

  manage frequency differences between a receiver that receives similar waveforms and a transmitter

  having the function of emitting them The waveforms are in real

  lity of the graphs of the tension, which is carried on the axis of the ordinates-

  born 10, as a function of time, plotted on the x-axis 12.

  The waveform 14 of FIG. 1 is an amplitude modulated signal which is formed by modulating an information signal on an electromagnetic wave, the amplitude (i.e. the voltage) of the form wave (14) varying according to the values of the information signal which modulates it The part of waveform 14 which contains the information is therefore limited to the amplitude of the form

  because the amplitude variations of the waveform 14 cor-

  respond to variations in the amplitude of the information signal.

  The amplitude of waveform 14, which is called the envelope of

  The waveform is represented in FIG. 1 by curve 16.

  Curve 16 has a shape analogous to that of the information signal.

  mation which, after modulation of an electromagnetic wave, forms waveform 14 The frequency of waveform 14 does not vary and corresponds to the frequency of the unmodulated wave on which the information signal has been modulated This frequency is called the

  carrier frequency of waveform 14, and the electro-

  magnetic is called the carrier wave.

  The waveform 18 of FIG. 2 A is a frequency modulated signal which is formed by modulating an information signal on an electromagnetic wave. The amplitude of the waveform 18 does not vary; however, the frequency of waveform 18 varies in

  function of the values of the information signal which modulates the

  variable quence of waveform 18 therefore constitutes the part of

  the waveform that contains the information The variation of

  However, the frequency of the waveform 18 caused by the modulation of the electromagnetic wave by the information signal is small, in comparison to the frequency of the electromagnetic wave. waveform 14 of the

  Figure 1, be characterized by the frequency of the electro- wave

  magnetic on which the information signal is modulated Lé; this frequency is called the carrier frequency of the waveform

  18, and the electromagnetic wave is called the carrier wave.

  The waveform 19 of FIG. 2 B is a phase modulated signal which is formed by modulating an information signal on an electromagnetic wave. The amplitude of the waveform 19 does not vary; however, the phase of the waveform 19 varies depending on the values of the information signal which modulates it. Thus, the variable phase of the waveform therefore constitutes the part of the waveform 19 which contains the formation. It should be noted that the abrupt phase variation of the waveform 19 of FIG. 2 B is not given

  as an illustration, and that the phase modulated signal presents

  In fact, a gradual phase variation The phase variation of waveform 19 does not fundamentally change the carrier frequency of the signal Therefore, once wave 19 has been modulated, we can say (as for waveform 14 in Figure 1 and waveform 18 in Figure 2 A) it

  is characterized by the carrier frequency.

  We now refer to the graphical representation of Figure 3, which represents the constellation of points in a diagram

  discrete coding code for coding an information signal.

  As indicated above, by coding an information signal in the form of a series of discrete coded signals, it is possible to transmit more than one signal at a particular frequency, which

  therefore significantly increases the capacity to transmit information

  tion of a particular frequency band.

  Figure 3 shows an eight-level phase shift modulation (PSK) system in which an information signal can take the form of any one of eight different levels (i.e., phases) D other diagrams of

  discrete coding are naturally also possible With this system

  teme, the information signal is coded according to two bit currents

  parallel, designated by I (t) and Q (t) At the sample instants-

  swim ti, the bit currents I (ti) and Q (ti) form a vector whose possible values (i.e. the tips of the vectors)

  are graphically represented in FIG. 3 The axis of the ordinates

  born 20 and the abscissa axis 22 are calibrated according to the ampli

  respective studies of Q (t) and I (t) It is possible to modulate such a vector on an electromagnetic wave so as to form a modulated information signal o the information content of the signal

  consists of a series of discrete levels (or phases).

  The coding scheme in Figure 3 illustrates the standard selected for digital cellular radiotelephone transmission systems to be implemented in the United States of America. With particular regard to the United States of America standard, only four differential changes between any two consecutive vectors are authorized. Such a coding scheme is called a modulation scheme by displacement of

  differential quadrivalent phase (DQPSK).

  FIG. 4 is a graphical representation of the waveform 18 of FIG. 2 A, as a function of the frequency. The ordinate axis 50 of FIG. 4 represents the power of the signal, indicated in milliwatts, as a function of the frequency, which is calibrated in hertz, on the abscissa axis 52 The waveform is centered on a central frequency fc 'designated by the reference number 54 The side bands 56 and 58 constitute the part of the waveform containing l information The bandwidth of the

  frequency modulated signal (FM) is indicated by a segment 59.

  We now refer to the graphical representation of FIG. 5, where a waveform on which a DQPSK signal is modulated is plotted, as a function of frequency.

  DQPSK is a composite modulated signal with both components

  amplitude modulation (as in Figure 1) and phase modulation components (as in Figure 2 B) The power of the waveform, expressed in milliwatts on the axis of

  ordinate 60 is plotted against frequency, which is expressed

  mée in hertz, on the abscissa axis 62 The signal is centered on

  a central frequency f indicated by a reference number 64.

  c The central frequency 64 defines the lateral bands 66 and 68 La

  DQPSK signal bandwidth is indicated by segment 69.

  Figure 6 is a graphical representation showing two adjacent transmit channels, where each transmit channel has a bandwidth of 30k Hz The dotted lines 74, 76 and 78 in Figure 6 indicate the respective boundaries of the adjacent channels 70 and 72, the line 79 indicating the border between the channels 70 and 72 As for the graphs in FIGS. 4 and 5, the waveforms plotted in FIG. 6 represent the power as a function of the frequency. For example, the shape wave traced

  in the transmit channel 70 is a frequency modulated signal

  with a central frequency 80 and side bands 82 and 84.

  As indicated above, the side bands 82 and 84 represent

  the part of the modulated information sign L which contains the information

tion.

  A certain frequency drift for the transmitted signal is authorized, as long as the side bands of the signal are maintained within the 30 k Hz bandwidth limits of channel 70 With regard to the analog frequency modulated signal which is traced in transmit channel 70, permissible frequency drift

  ble is indicated by an arrow 86 drawn above the fr

  central quence 80 The arrow 86 illustrates the admissible position of the central frequency 80 of the signal when the latter continues to be maintained within the borders of the channel 70 A drift of the carrier wave of the modulated information signal which does not not exceed, in frequency, the drift indicated by the arrow 86 maintains the lateral bands 82 and 84 inside the bandwidth of K Hz of the channel 70 In quantitative terms, the authorized frequency drift for the signal is approximately 2 100 Hz on both sides

  of The central frequency The signal centered on the central frequency

  trale ftc, which is indicated by the reference number 80 ', shows a frequency modulated signal which has drifted towards the higher frequencies, while remaining within the admissible limits for the

  frequency drift The side bands 82 'and 84' are now

  bare between borders 74 and 76 of transmission channel 70 It should however be noted that any additional increase in drift

  of the waveform would cause the sideband 84 'to extend beyond

  beyond the border 76 which separates the transmission channels 70 and 72.

  This drift would cause the channel signals to overlap

  adjacent and thus cause mutual interference.

  The transmission channel 72 shown on the right-hand side of FIG. 6 defines a 30 k Hz transmission channel extending between the dotted lines 76 and 78 By way of example, an information signal has been placed at DQPSK modulation within the transmission channel 72 Like the signal in FIG. 5, the modulated information signal is centered on a central frequency fc 'indicated by the reference number 90 The central frequency 90

  defines side bands 92 and 94 The frequency drift auto-

  laughed at for a discrete signal such as the signal illustrated in the

  transmit channel 72 is less than the auto frequency drift

  laughing at a classic analog signal (such as the illus-

  very in the transmission channel 70) The arrow 96 drawn above

  of the central frequency 90 illustrates the automatic frequency drift

  laughing at the discrete coded signal of the transmission channel 72 The arrow

  96 corresponds to the arrow 86 drawn above the crest of

  pu Lsion 80 of transmit channel 70, and it defines the drift of

  authorized frequency for the discrete coded signal In quantitative terms

  frequency drift allowed for the coded signal distributed

  peak is around 200 Hz on either side of the center frequency

  tra Ie This authorized drift is lower, of an order of large

  deur, to that authorized for an analog and classic signal.

  The much smaller frequency drift which is self-

  laughing at a DQPSK signal is due not only to the signal passing through adjacent channels, but also to the fact that the circuit

  receiver used to receive and demodulate DQPSK cannot receive

  view and demodulate a DQPSK signal with as much precision when

that it is shifted in frequency.

  While a frequency command (i.e. a ver-

* rusting) is advantageous for minimizing the frequency drift of a conventional analog signal (and is sometimes necessary

  to prevent overlapping of signals), the frequency control

  quence is virtually always necessary when transmitting certain

  some discrete coded signals Digital signal processors can be constructed which make it possible to obtain an indication of the central frequency (or of another reference frequency) of any signal transmitted, whether this is a conventional analog signal or a coded signal discrete However, the power consumption of a digital signal processor is important When transmitting discrete coded signals, a digital signal processor must

  operate only intermittently.

  We now go to the diagram in Figure 7, which shows

  understands in the form of a functional block diagram the elements of the frequency control system according to the invention.

  frequency control constituting an embodiment of the

  The function of vention is to determine the center frequency, or another reference frequency, of a coded modulated information signal.

  discrete or a modulated analog modulated information signal

  A digital signal processor is only active when the information signal is a discrete coded signal, and moreover only when the discrete coded signal is received, which minimizes the

  processor power consumption.

  Alternatively, it is possible to use,

  when receiving an analog information signal, the pro-

  digital signal arrester to determine the reference frequency

of the signal emitted.

  The transmitted signal, whether a conventional analog signal or a discrete coded signal, is transmitted to an antenna

  (or other electromagnetic wave receiving device) 110.

  The signal received by the antenna 110 is filtered and amplified, if necessary.

  saire, and it is supplied to a first low-down converter circuit

  sor 112 The step-down converter circuit 112 converts the signal having the transmission frequency (which can for example be

  890 M Hz) into a lower frequency signal, for example 45 M Hz.

  The step-down converter circuit 112 produces the frequency signal

  lower quence on a line 114, which is connected to a second step-down converter circuit 116 The second circuit

  step-down converter 116 converts the signal to it

  via line 114 as a baseband signal The circuit con-

  step-down converter 116 produces a signal in phase on a line 118 and a signal in phase quadrature with it on a line 120 The phase signal produced on line 118 is supplied to a baseband filter 122 and The signal in quadrature produced on line 120 is supplied to a baseband filter 124 The step-down converter circuit 116 and filters 122 and 124

  can together constitute a single integrated circuit chip, ap-

  eg The zero intermediate frequency circuit (ZIF), which is indicated

  by block 126 in broken lines.

  The filtered signals obtained using filters 122 and 124 are produced respectively on lines 128 and 130 Filters 122 and 124 have passbands allowing passage

signals of desired frequencies.

  When the antenna 110 receives a discrete coded signal, the

  filtered signals produced by filters 122 and 124 are respected

  Supply to analog-to-digital (A / D) converters 132 and 133 The digital signals produced by the A / D converters 132 and 133 are supplied to a DSP 134 digital signal processor via lines 136 and 137 The digital signal processor processes the digital signals supplied to it and produced on

  a line 138 an audio signal which is indicative of the information signal

  mation transmitted in discrete coded form to the antenna The digital signal processor 134 also produces on a line 140 an output signal which is indicative of the center frequency, or of another reference frequency, of the signal transmitted The signal product on line 140 can be used to lock the receiver frequency to the center frequency, or another

  reference frequency, of the signal emitted.

  When the signal sent to the antenna 110 is a conventional analog signal, the filtered signals produced by the filters 122 and 124 are delivered to a step-up converter circuit 142 The step-up converter circuit 142 converts the filtered signals in phase and in quadrature respectively phase produced on lines 128 and 130 in a higher frequency signal, and this signal is supplied on line 143 The signal

  supplied on line 143 is sent to the demodulation circuit 144.

  The demodulation circuit 144 demodulates the signal it received using conventional frequency demodulation techniques. Conversion of the baseband signals produced on lines 128 and 130 into a higher frequency signal is necessary to

  allow demodulation by a conventional demodulation circuit.

  The demodulation circuit 144 produces, on line 145,

  an audio signal which is representative of the information signal part

  mation of a conventional analog modulated signal received by the antenna The signal produced by the step-up converter circuit 142

  on line 143 is also supplied to a phase detector 146.

  The phase detector 146 compares the frequency of the output signal of the step-up converter circuit 143 with the frequency of the signal

  produced by the offset loop circuit 147 so as to pro-

  draw on line 149 an output signal which is indicative of the center frequency, or of another reference frequency, of the

signal issued.

  Since a transmitter which transmits a modulated information signal and which, in the particular case of a cellular transmission system, comprises a base station generally has a size which allows it to include a means making it possible to prevent

  frequency drift due to ambient conditions and irregularities

  tides of voltage, center frequency, or other frequency

  reference frequency, of the signal emitted can be used as a frequency

  reference frequency by the receiver.

  We now refer to FIG. 8 which, partially in the form of a block diagram and partially in the form of

  circuit, shows a preferred embodiment of the communication system

  frequency command of the invention A modulated information signal, which is either a conventional analog signal or a coded signal

  discrete, is transmitted by the transmitter 150 and is received by the antenna 152.

  The antenna 152 supplies the signals received to the filter 156 via a line 154 The filter 156 has a pass band making it possible to pass signals whose frequencies lie within a desired frequency range, on a line 258 Les signals allowed to pass through the filter-156 are supplied to a mixer circuit 160 which acts as a step-down converter for the modulated signal received by the antenna 152 The mixer 160 receives on a line 162 an oscillating signal produced by an oscillator at

  voltage control (VCO) 164 The voltage control oscillator

  sion 164 forms part of a conventional phase lock loop (PLL) circuit comprising a phase detector 166, a filter 168 and division circuits (+ N) 170 and (+ R) 172 The mixed signal produced by the mixer 170 is supplied via the line

  163 to a filter 165 The filter 165 has a per-

  putting to pass wanted frequency signals on a

  line 166 Line 166 is coupled to the frequency input

  middle (IF) of a circuit 180 with zero intermediate frequency section (ZIF) The mixer 160, the oscillator 164 and the associated PLL circuit, as well as the filter 165, are contained in a block 112,

  indicated by a broken line, which corresponds to the conver-

  step-down weaver 112 shown in the block diagram of the

figure 7.

  A circuit forming a second PLL circuit, which is

  consisting of a voltage-controlled oscillator (VCO) 182, a low-pass filter 184, a phase detector 186 and division circuits (+ N) 188 and (+ R) 190, provides a signal oscillating at a second LO input (local oscillator) of circuit 180 The second PLL circuit and circuit 180 are contained in a block 126, indicated by a dashed line, which corresponds to the circuit of

  intermediate frequency zero 126 of FIG. 7.

  A reference oscillator 192 produces an oscillating signal on a line 194 which is divided by the divider circuit (+ R) 196 so as to provide a reference oscillating signal on the intermediate frequency reference input (IFREF) of the circuit 180 La line 194 is also coupled to the oscillator circuit connected to the second LO input of circuit 180 and to the PLL circuit which produces an oscillating signal on line 162 intended for the mixer, so that an oscillating signal is supplied by it

each of the PLL circuits.

  The intermediate frequency section circuit 180 pro-

  duit IQ output signals respectively on lines 198 and when the modulated signal received by the antenna 152 is constituted

  of a discrete coded signal When the modulated signal received by the an-

  tenne 152 consists of a classic analog signal, the circuit

  baked at intermediate frequency section (SECTION IF) 180 produces an audio output signal (FM AUDIO) on line 202 Line 202

  is coupled to an audio processing circuit (not shown).

  When the antenna 152 receives the conventional analog modulated signal, the circuit 180 also produces a phase detection output signal (PHASE), which is applied to a filter 204. The filter 204 produces on the line 206 a filtered output signal. indicative of the

  phase, or frequency, of the signal received by the antenna 152.

  When circuit 180 produces output signals I and Q on lines 198 and 200, the output signals produced are respectively supplied to analog-digital converters (A / D) 208 and 210 The A / D converters 208 and 210 provide of

  digital signals to the digital signal processor (DSP) 212.

  While Figure 8 shows parallel connections between converters 208 and 210 and processor 212, it should be noted that

  serial connections are also possible.

  The processor 212 also receives phase angle or frequency information, indicated by the filtered signal produced by the filter 204, which signal is supplied to the processor 212 via an analog-digital converter (A / D) 214 The processor of digital signals 212 processes the I and Q signals and the frequency information supplied to them and produces output signals which are converted into an analog signal by the converter

digital-analog (D / A) 216.

  The analog signal produced by the D / A converter 216

  is supplied via a line 218 to a frequency control switch

  quence 220 of the automatic gain control type Switch 220 is actuated by an external signal, which is supplied to it via a line 222, having the effect of alternately connecting the lines

  206 and 218 (respective inputs A and B) to the reference oscillator

  rence 192 The signals supplied via lines 206 and 218 are indi-

  catives of the frequency of the signals received by the antenna 152 As indicated above, the signal present on line 206 is indicative

  the frequency of the signal received when it is an analog signal

  classical logic, and the signal supplied via line 218 is indi-

  cative of a discrete coded signal received by the antenna 152 The signals respectively supplied on Lines 206 and 218 are used to

  change the oscillation frequency of the 192 Plus oscillator by-

  in particular, the variations in the frequency of oscillator 192

  correspond to variations in the frequency of the signal emitted at

antenna 152.

  The switch 220 can for example be made up of any electronically controlled switch, or of another type. For example, the switch 220 can be made up of transmission doors of the CMOS (metal-oxide-semiconductor) type.

  complementary) arranged as a 2: 1 multiplexer.

  We now go to the representation of Figure 9,

  which, partially in the form of a block diagram and partially-

  ment in circuit form, shows another embodiment

  preferred of the invention In the same way as for the embodiment

  sation of Figure 8, a signal, which is either an informational signal

  Conventional analog modulated signal, ie a discrete coded modulated information signal, is transmitted by a transmitter 250 and is received by the antenna 252 The antenna 252 supplies the signals received to a bandpass filter 256 via a line 254 The filter 256 has a bandwidth which lets the signals belonging to a desired frequency range pass over a line 258 to a mixer 260 which acts as a step-down converter for the modulated signals received by the antenna 252 The mixer 260 receives via a line 262 an oscillating signal produced by a voltage controlled oscillator (VCO) 264 The voltage controlled oscillator 264 constitutes a part of a conventional PLL circuit comprising a phase detector 266, a filter 268 and circuits dividers (+ N) 270 and (+ R) 272 The mixed signal obtained from the mixer 260 is supplied via a line 274 to a bandpass filter 276 The filter 276

  has a bandwidth that lets the signals of

  quences wanted via a line 278 bound for the entry of

  intermediate frequency (IF) of an intermediate frequency circuit 280.

  A circuit forming a PLL, which consists of an oscil-

  voltage-controlled reader (VCO) 282, low-pass filter 284, phase detector 286 and divider circuits (+ R) 288 and (+ N) 290, produces an oscillating signal for a second

  LO input (local oscillator) of circuit 280.

  A reference oscillator 292 produces on a line 294 an oscillating signal which is divided by the divider circuit 296 and is supplied to an intermediate frequency reference input

  (IFREF) of circuit 280 Line 294 is also coupled to the os-

  cillator connecting the second LO input of circuit 280 and to the

  PLL circuit which produces an oscillating signal on line 262 below

  mixer 260 to provide an oscillating signal to

each of the PLL circuits.

  The intermediate frequency section circuit 280 produces output signals I and Q on respective lines 298 and 300, when the modulated signal received by the antenna 252 consists of a discrete coded signal When a modulated information signal received by the antenna 252 is constituted by a conventional analog signal, the intermediate frequency section circuit 280 produces an audio output signal (FM AUDIO) on a line 302 The line 302 is coupled to an audio processing circuit (not shown) When the antenna 252 receives a conventional analog modulated information signal, circuit 280 also produces a phase detection output signal (PHASE) which is supplied to a filter 304 The filter 304 produces on a line 306 a filtered output signal which is indicative of the phase, or frequency, of the signal received by

antenna 252.

  When the circuit 180 produces the output signals I and Q on the respective lines 298 and 300, the output signals I and Q thus formed are respectively supplied to analog-digital converters (AID) 308 and 310 The A / D converters 308

  and 310 produce digital signals to a process

  Digital signal processor (DSP) 312 While figure 9 illustrates parallel connections between the converters 308 and 310 and the processor 312, it should be noted that serial connections are also possible The digital signal processor 312 processes the signals I and Q supplied thereto and produces output signals which are converted to an analog signal by a digital to analog converter (D / A) 316 The analog signal produced by the digital to analog converter 316 is supplied

  via a line 318 to the reference oscillator 292.

  The embodiment of FIG. 9 differs from that of FIG. 8 in that, instead of the signals produced on the

  lines 306 and 318 are supplied to a switch, the signal pro

  duit on line 318 is directly applied to the reference oscillator 292, and the signal supplied by line 306 is delivered to

  an offset voltage controlled oscillator (OFF VCO) 320.

  The offset oscillator 320 produces on a line 322 a signal

  oscillating which is supplied to an image rejection mixer 324.

  This reference oscillator 192 is maintained at a pre-

  established, and the LO loop is locked to the frequency of the reference oscillator 292 When the antenna 252 receives a conventional analog modulated information signal, the digital signal processor 312 is disabled, and the I and Q signals are not not used; instead, the phase detection output signal produced by circuit 280 is supplied to filter 304 The filtered signal produced by filter 304 is supplied via line 306 to the offset oscillator 320, which, when is connected in the illustrated way, modifies the oscillation frequency supplied to the second LO input Conversely, when the antenna 252 receives a discrete coded modulated information signal, the oscillator 320 is disabled, which causes an imbalance in the mixer image rejection 324 so that the second LO feedback signal passes through circuit 324, and the second LO signal divided by circuit N is reprogrammed This locks the second LO signal on the frequency of the reference oscillator 292, and the digital signal processor 312 produces a control signal on the line

  318 to change the frequency of oscillator 292 to match

  weighting with the frequency of the signal received by the antenna 252.

  The reference oscillator 192 of FIG. 8 and the oscillator-

  reference lator 292 of FIG. 9 can for example be constituted by a current controlled oscillator when the signals supplied on lines 206 and 218 and on line 318 are

  Current signals Another possibility is that the oscillations

  teurs 192 and 292 can be constituted by oscillators controlled by data When the signals supplied on Lines 206 and 218 and on La Liane 318 are constituted by data signals A reference oscillator controlled by data is advantageous in that the noise produced on a Line of

  command cannot cause variations in frequency.

  Of course, the Man of Art will be able to imagine,

  from the frequency control system, the description of which

  has just been given for illustrative purposes and is in no way limitative, various variants and modifications not leaving the

framework of the invention.

Claims (8)

  1 Frequency control system allowing cor-   to regulate frequency differences between a receiver, having the function of receiving a modulated information signal on a carrier wave characterized by a certain frequency in order to form a modulated information signal, and a transmitter which transmits the information signal modulated, said frequency control system being characterized by: means (152-222, 252-292), comprising at least one variable frequency oscillator (192, 292, 320) which oscillates on an oscillation frequency, and serving receiving the modulated information signal;   means forming an analog signal detection circuit   logic (180, 280) which is used to determine the particular frequency characterizing the carrier wave when the information signal modulated thereon is an analog signal, and used to produce   a first frequency reference signal indicative of the frequency   particular quence thus determined;   means forming a signal detection circuit   cret (212, 312) used to determine the particular frequency characterizing the carrier wave when said information signal   modu Lé on it is a discrete coded signal used to pro-   drawing a second frequency reference signal indicative of the particular frequency thus determined; and   means (206, 306, 218, 318) for modifying the frequency   oscillation frequency of the variable frequency oscillator (192, 292, 320), which responds to values of the first frequency reference signal or the second frequency reference signal by correcting the frequency differences between the receiver and the 'transmitter.   2 Frequency control system as claimed   tion 1, characterized in that it further comprises a means forming a switch (220) used for switching between the means which forms a circuit for detecting an analog signal (180), in the case where the signal d the information is constituted by the analog signal, and the means forming a discrete signal detection circuit (212) in the case where the information signal is   consisting of a discrete coded signal.   3 Frequency control system as claimed   tion 1, characterized in that the means forming a detection circuit   Discrete signal (212, 312) can only work when   the information signal consists of the discrete coded signal.   4 Frequency control system according to claim-   tion 3, characterized in that the first frequency reference signal produced by the means forming an analog signal detection circuit (180, 280) comprises a control voltage produced by a phase detector which forms part of the means providing demodulation.   Frequency control system as claimed in claim 4, characterized in that the means forming a discrete signal detection circuit (212) produces a signal of a preset value when the information signal consists of a   signal other than the discrete coded signal.   6 Frequency control system according to claim-   tion 5, characterized in that the means forming a detection circuit   discrete signal (212) produces a signal in response to the voltage   control signal produced by the phase detector of the demodulation means of the reception means when the information signal   consists of the signal other than the discrete coded signal.   7 Frequency control system according to claim-   tion 1, characterized in that the means forming a detection circuit   The analog signal (280) further includes a frequency demodulation circuit having a loop circuit (282-290, 320324) for determining the frequency of teuse. offset the wave carries   8 Frequency control system according to claim-   tion 7, characterized in that said means forming a discrete signal detection circuit (212, 312) comprises a processor digital signals (212, 312).   9 Frequency control system according to claim-   tion 1, characterized in that said receiving means comprises a   reference variable frequency oscillator (292) and an oscillating   variable frequency offset shifter (320).   Frequency control system as claimed   tion 9, characterized in that said first frequency reference signal produced by the means forming an analog signal detection circuit (280) is applied to the variable offset frequency oscillator (320) and the second frequency reference signal   frequency (318) produced by the means forming a detection circuit   The discrete signal (312) is applied to the frequency oscillator.   quence reference variable (292).